1. Field of the Invention
The present invention relates generally to a scroll compressor and, more particularly, to an oil circulation arrangement in the scroll compressor.
2. Description of Related Art
A scroll compressor featuring less vibrations and less noise has a suction chamber formed radially outwardly of two opposing scroll elements defining a plurality of volume-variable, sealed working pockets therebetween, and also has a discharge port formed centrally of the scroll elements. The scroll compressor allows a compression fluid to flow in one direction, and does not require a discharge valve for compressing the fluid, unlike a reciprocating compressor or a rotary compressor. It is also generally known that the scroll compressor has a constant compression ratio and generates relatively small discharge pulsations without requiring a large discharge space.
For further improving vibration and noise characteristics, a structure as shown in FIGS. 8 and 9 has been proposed to lessen a jumping phenomenon of an orbiting scroll during high-speed operations.
As shown in FIGS. 8 and 9, an orbiting scroll 1001 has an orbiting end plate 1001a coupled to a drive pin 1007a of a drive shaft 1007, while a stationary scroll 1002 opposing the orbiting scroll 1001 similarly has a stationary end plate 1002a. An outer peripheral portion of the orbiting end plate 1001a is slidably accommodated within a space defined between the stationary end plate 1002a and a frame 1008 carrying one end of the drive shaft 1007, to thereby prevent inclination of the orbiting scroll 1001 relative to the longitudinal direction of the drive shaft 1007 or jumping of the orbiting scroll 1001 in this direction when the compression load or the inertia forces of the rotating members change, for example, when the compressor starts, stops or operates at a high speed. An axial gap between the orbiting scroll 1001 and the stationary scroll 1002 is accordingly ensured to tightly seal a compression chamber, thus enhancing the compression efficiency. At the same time, abnormal noise or vibrations, which have been hitherto caused by an undesirable collision of the compressor elements, are reduced, and a decrease in durability of the sliding portions is avoided in the illustrated structure.
Moreover, with the aim of improving sealing properties of the compression chamber, the fluid in the compression chamber is introduced into a back chamber of the orbiting scroll 1001 on the opposite side to the compression chamber, so that the orbiting scroll 1001 may be pressed towards the stationary scroll 1002 by the fluid pressure in the back chamber to prevent the orbiting scroll 1001 from moving away from the stationary scroll 1002. This kind of construction is disclosed in, for example, Japanese Laid-Open Patent Publication (unexamined) No. 55-142902 or U.S. Pat. No. 3,994,633.
Japanese Patent Publication No. 5-67796 discloses a method of pressing the orbiting scroll toward the stationary scroll. According to this publication, a lubricating oil in a discharge chamber is introduced into a back chamber of the orbiting scroll on the side thereof opposite to the compression chamber.
It is generally known that the scroll compressor has a constant compression ratio and, hence, the pressure in the compression chamber when fluid compression is completed is determined by the suction pressure.
However, immediately after a completely compressed gas is discharged out from the discharge port to the discharge chamber, the pressure in the compression chamber in the vicinity of the discharge port becomes equal to that of the discharge port or discharge chamber.
As such, the actual pressure distribution in the compression chamber is influenced and changed by the pressure of the discharge chamber. Particularly, if the pressure in the discharge chamber is extraordinarily larger than the pressure of the compressed gas, even the pressure of the compression chamber near the suction chamber is influenced by the pressure of the discharge chamber as a result of a back flow of the gas from the discharge chamber and a leak of the gas from the compression chamber. Further, when the compressor operates at a low speed at which the leak rate from the compression chamber is relatively high and when the pressure in the discharge chamber is low, the pressure of the completely compressed gas becomes close to the pressure of the discharge chamber. In other words, the actual pressure in the compression chamber is influenced by the pressure in the discharge chamber.
Even when the back pressure to the orbiting scroll is so set as to properly press the orbiting scroll toward the stationary scroll upon a specific selection of the suction and discharge pressures, it is likely that this back pressure will become excessive or insufficient during actual driving of the compressor. Consequently, the problem arose that the orbiting scroll axially moves away from the stationary scroll, or is excessively pressed toward the stationary scroll, resulting in a reduction of the compression efficiency due to the leak of the compressed gas, or an increase of inputs and deterioration of the durability due to an increasing frictional resistance at the sliding portions.
To overcome this problem resulting from an unstable axial gap of the compression chamber, U.S. Pat. No. 4,395,205 discloses that the gas pressure in the compression chamber acting on the orbiting scroll is supported by a thrust bearing surface defined on a stationary member, without providing a back chamber on the side of the orbiting scroll opposite to the compression chamber. The thrust bearing surface is supplied with a lubricating oil.
This construction, however, introduces a large frictional loss between the orbiting scroll and the thrust bearing. It is also difficult to ensure a desired axial gap of the compression chamber at low cost, resulting in a large leakage rate of the compressed gas. For this reason, the compressor of the construction referred to above, particularly, that of a small capacity, has a low compression efficiency.